The Concentration of Substances Year 10 Physical Science Chemical Analysis Unit

Concentration Concentration is usually defined in terms of the amount of solute per volume of solvent. The greater the quantity of solute added to the solvent, the more concentrated the solution becomes. When there is only a very small amount of solute present, the solution may be called dilute. A concentrated solution may be diluted by the addition of more solvent.

Concentration

Conc. (C) = mass of solute (m) / volume of solution (V) Expressing Concentration There are many ways to express concentration units. A common form is the ratio of the units of solute to the units of volume (e.g. g/L and mg/L). Conc. (C) = mass of solute (m) / volume of solution (V) A fencepost preservative solution is prepared by dissolving 4.00 g of zinc chloride in enough water to make 2250 mL of solution. Find the concentration (C) of this solution in g/L and then in mg/L.

Expressing Concentration V = 2250 mL = 2250 / 1000 = 2.25 L Concentration = mass / volume Concentration = 4.00 g / 2.25 L Concentration = 1.78 g/L zinc chloride Since there are 1000 mg in a 1 g Concentration = 1.78 x 1000 mg/L Concentration = 1780 mg/L zinc chloride

% w/w = mass of solute / mass of solution x 100% Percentage by Mass (% w/w) Percentage by mass describes the mass of solute, measured in grams, present in 100 grams of the solution. % w/w = mass of solute / mass of solution x 100% Calculate the concentration (%w/w) for a solution of 38.4 g of NaCl in 150.0 g of solution % w/w NaCl = 38.4 g / 150 g x 100 % w/w NaCl = 25.6 %

% v/v = volume of solute / volume of solution x 100% Percentage by Volume (% v/v) The abbreviation v/v indicates that the percentage is based on volumes of both solute and solution. The same units must be used to record both volumes. % v/v = volume of solute / volume of solution x 100% A particular brand of champagne has an alcohol concentration of 11.5% v/v. What is the alcohol content of a 750 mL bottle? volume solute = % v/v . volume solution volume alcohol = (11.5 /100) x 750 mL volume alcohol = 86.25 mL

% m/v = mass of solute / volume of solution x 100% Percentage Mass / Volume (% m/v) Percentage mass/volume describes the mass of solute, measured in grams, present in 100 millilitres of the solution. % m/v = mass of solute / volume of solution x 100% Find the % w/v of a saline solution consisting of 4.30 g of sodium chloride dissolved in 0.500 L of aqueous solution. V = 0.500 L = 500 mL % w/v saline = 4.30 g / 500 mL x 100% % w/v saline = 0.86 %

Parts Per Million (ppm) When very small quantities of solute are dissolved in a solution, the concentration can be measured in parts per million (ppm). This unit can be written as mg/kg, mg/g, mg/L and mg/mL. If 10 g of chlorine gas are dissolved in every 2 500 000 L of water, express the concentration of the chlorine water in ppm (μg/g). m (chlorine) = 10 g = 10 x 106 mg = 1 x 107 mg. m(water) 2,500,000 L ≈ 2 500 000 kg = 2 500 000 000 g Concentration = 1 x 107 mg / 2.5 x 109 g = 4.0 x 10-3 mg/g

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